1. Technical Field of the Invention
This invention relates generally to electromagnetic transducers such as audio speakers, and more specifically to a motor structure for such, having one or more soft magnetic members with a laminated construction.
2. Background Art
FIG. 1 illustrates a conventional speaker 10 with an external magnet geometry motor structure 12 driving its diaphragm assembly 14. The motor structure includes a pole plate 16 style yoke, made of soft magnetic material and including a back plate 18 and a pole piece 20 that are either magnetically coupled or of integral construction. The pole plate may optionally include a ventilation hole 22 for depressurizing the diaphragm assembly. One or more external ring hard magnets 24 are magnetically coupled to the back plate. A top plate 26 of soft magnetic material is magnetically coupled to the hard magnets. A magnetic air gap 28 is formed between the top plate and the pole piece.
The diaphragm assembly includes a basket 30 which is mechanically coupled to the motor assembly to support the other, moving parts of the diaphragm assembly. A diaphragm 32, sometimes referred to as a cone, is coupled to the basket by a flexible suspension component known as a surround 34. A voice coil former or bobbin 36 is mechanically coupled to the diaphragm, and is coupled to the basket by a flexible suspension component known as a spider 38. The surround and spider allow the bobbin and diaphragm to move axially with respect to the motor structure, but prevent, as much as possible, their lateral movement and rocking. An electrically conductive voice coil 40 is wound around and mechanically coupled to the bobbin, and is disposed within the magnetic air gap of the motor structure. A dust cap 42 is coupled to the diaphragm to seal the open end of the bobbin.
FIG. 2 illustrates a conventional speaker 50 with an internal magnet geometry motor structure 52 driving the diaphragm assembly 14. The motor structure includes a yoke or cup 54 of soft magnetic material. One or more internal hard magnets 56 are magnetically coupled to the cup, and an internal top plate 58 of soft magnetic material is magnetically coupled to the hard magnets, forming a magnetic air gap 60 between the top plate and the cup. The motor structure may be ventilated, as shown, or it may be unventilated and have disc magnets and a disc top plate, rather than the ring configuration shown.
Both the external magnet geometry motor structure of FIG. 1, and the internal magnet geometry motor structure of FIG. 2 are axisymmetric, meaning that they have a generally circular shape when viewed along their motors' respective axes. In order to use larger magnets, it has previously been necessary to grow the motor structure in all radial directions, by making the diameter larger.
FIG. 15 illustrates the conventional external magnet motor structure 10 (of FIG. 1), shown in an isometric sectional view with multiple cutaways. The motor structure includes a pole plate including a back plate 18 and a pole piece 20. A pair of magnets 24 are magnetically coupled to the pole plate, and a top plate 26 is magnetically coupled to the magnets. The space between the inner diameter of the top plate and the pole piece defines a magnetic air gap 28.
One significant drawback in the prior art is that the alternating current in the voice coil (not shown) will induce eddy currents (denoted EC and shown in heavy, curved arrows) in various of the motor components, including the pole piece. These eddy currents cause inductive heating of the motor components through which they are carried. The eddy currents themselves cause magnetic fields (flux modulation) which tend to counteract the desired operation of the motor assembly, degrading sound quality, increasing distortion, and reducing high frequency output, by increasing the inductance of the transducer. Worse still, these effects become more pronounced, the harder the speaker is driven.
In the unrelated field of transformers, it is known to avoid eddy currents by fabricating the outer magnetic circuit core as a laminate, with multiple separate members coupled together, rather like plywood. Adjacent members are electrically insulated from each other, but are magnetically coupled to each other; the overall laminated structure is highly magnetically conductive, but is minimally electrically conductive. Ideally, the lamination should be perpendicular to the direction in which eddy currents would tend to be induced; that is, the eddy current paths will be interrupted by an electrical insulation layer between each magnetically conductive lamination.